This invention relates to a thermal time-delay switch and more particularly to a switch with switching contacts operable by means of bimetallic elements.
In an automobile wth an electronic fuel control device, an electronic control system for the device operates to make constant the air-fuel ratio of a mixture to be supplied to the engine. When starting the engine, it is necessary to supply a mixture of a high fuel concentration and hence, the fuel supply upon the engine start is carried out through a fuel supplying conduit independent of the electronic control system. The fuel supplying conduit is provided with an electromagnetic valve which is opened for a certain period of time only when the engine is started. Namely, the electromagnetic valve is opened at the time that a starter switch is closed but the valve is closed in about ten seconds after closing the starter switch even if the starter switch is kept closed.
FIG. 1 shows a circuit in which a thermal time-delay switch heretofore in use is used for controlling an electromagnetic valve operable when the engine of the aforementioned automobile is started. A thermal time-delay switch 10 comprises a bimetallic element 12, a heating resistor 14 and contacts 16 and 18. The heating resistor 14 wound on the bimetallic element 12 controls a time at which the contacts 16 and 18 separate. The contact 16 is directly mounted on the bimetallic element so that this bimetallic element serves as a conductor. The contact 18 is connected to one end of the heating resistor 14 and to one polarity of a battery 20.
The thermal time-delay switch 10 is connected to the other polarity of the battery 20 through a starter switch 22 to control the excitation of a coil 24 of electromagnetic valve. Namely, the other end of the heating resistor 14 is connected to the starter switch 22 and the coil 24 is connected between this juncture and the bimetallic element 12, that is, the contact 16. As far as the starter switch 22 is turned on and the contacts 16 and 18 are in engagement, an electrical current is passed through the coil 24.
The battery 20 as a voltage source feeding the thermal time-delay switch 10 is of 12 volts usually but its voltage decreases to 6 volts or less than 6 volts at a low temperature of about -- (minus) 30.degree. C. Under this condition, as shown at curve I in FIG. 2, it takes about 110 seconds for the contacts 16 and 18 to be separated from each other after the starter switch 22 is turned on, amounting up to about eleven times as long as a time delay of 10 seconds for 12 volts. If the bimetallic element 12 is designed to have rapid thermal response characteristics with a battery voltage of about 6 volts, in spite of the time delay which is desirably about 10 seconds with a normal battery voltage of 12 volts, it becomes short and undesirable. For this reason, it was inevitable to use a thermal time-delay switch having such a characteristic as shown at curve I in FIG. 2.
In electronic fuel injection control apparatus for the combustion engine of automobile with a thermal time-delay switch of the curve I characteristic, a highly concentrated mixture is injected into the engine upon the engine start, as a result, an exhaust gas is produced which contains a large amount of unburned components. If this undersirable state is prolonged, the ignition plug is wetted and the engine stops operating. Under these conditions, if the starter switch is restarted, a highly concentrated mixture is again injected into the engine, as a result, not only it is impossible to repeat the ignition of the engine but also sometimes a water hammer phenomenon causes the engine to be deformed or to be broken.